Re: Unstable select_parallel regression output in 12rc1

Christoph Berg <myon(at)debian(dot)org> writes:
> Building the 12rc1 package on Ubuntu eoan/amd64, I got this
> regression diff:

The append-order differences have been seen before, per this thread:

https://www.postgresql.org/message-id/flat/CA%2BhUKG%2B0CxrKRWRMf5ymN3gm%2BBECHna2B-q1w8onKBep4HasUw%40mail.gmail.com

We haven't seen it in quite some time in HEAD, though I fear that's
just due to bad luck or change of timing of unrelated tests. I've
been hoping to catch it in HEAD to validate the theory I posited in
<22315(dot)1563378828(at)sss(dot)pgh(dot)pa(dot)us>, but your report doesn't help because
the additional checking queries aren't there in the v12 branch :-(

> 12:06:27 @@ -103,7 +103,7 @@
> 12:06:27 -----------------------------------------------------
> 12:06:27 Finalize Aggregate
> 12:06:27 -> Gather
> 12:06:27 - Workers Planned: 1
> 12:06:27 + Workers Planned: 3
> 12:06:27 -> Partial Aggregate
> 12:06:27 -> Append
> 12:06:27 -> Parallel Seq Scan on a_star

We've also seen this on a semi-regular basis, and I've been intending
to bitch about it, though it didn't seem very useful to do so as long
as there were other instabilities in the regression tests. What we
could do, perhaps, is feed the plan output through a filter that
suppresses the exact number-of-workers value. There's precedent
for such plan-filtering elsewhere in the tests already.

regards, tom lane